Fluoropolymers are generally made by an aqueous dispersion process, which provides a suitable heat sink for controlling the heat of polymerization and can produce a high yield and high molecular weight relative to polymerization conducted in an organic solvent. In order to achieve stable dispersion or emulsion, a suitable surfactant or emulsifier must be employed. Fluorinated-surfactants are generally used because they can yield stable particle and high molecular weight fluoropolymers. However, the fluorinated-surfactants typically used in emulsion polymerization of fluoropolymers, such as the ammonium salt of perfluoro octanoic acid or salts of perfluoro alkyl sulfonic acids are expensive. They also present an environmental concern related to bio-persistence.
It is therefore desirable to carry out an aqueous dispersion polymerization of fluoropolymers in the absence of fluorinated-surfactants, without compromising the properties of the resultant fluoropolymers. It would also be desirable to produce small particle size emulsion so that latex stability during storage as well as quality of film formation is improved. Furthermore, it would be desirable to produce a dispersion and/or fluoropolymer resin having fewer extractable ions and extractable low molecular weight polymers, while generally yielding fluoropolymers that have similar or even improved properties compared to analogous fluoropolymers made in presence of added fluorinated-surfactants.
An aqueous dispersion polymerization is used as a means to control the thermal and viscosity problems associated with producing fluoropolymers. An aqueous dispersion consists of a discontinuous fluoropolymer phase dispersed throughout a water phase. Examples of aqueous dispersion polymerization include but not limited to emulsion and suspension polymerizations.
Emulsion polymerization of vinylidene fluoride (VF2) at moderate pressures and temperatures using fluorinated surfactants, free radical initiators, and trichlorofluoromethane as chain transfer agent is taught in the U.S. Pat. No. 4,569,978 in which (VF2) based polymers are produced with reduced tendency to generate cavity and greater resistance to discoloration at elevated temperatures. The process was refined in the U.S. Pat. No. 6,794,264 wherein particularly ozone depleting agent (trichlorofluoromethane) was replace by propane which is environmentally friendly chemical. It is noteworthy that in both processes fluorinated surfactant was needed to produce stable emulsion. For example, perfluorocarboxylate salts was used to stabilize fluoropolymer emulsion polymerizations, with the most common example being ammonium perfluorooctanoate or ammonium perfluoronanoate. The high degree of fluorination is thought to be necessary to prevent chain transfer reaction between a growing polymer chain and the surfactant which in turn may result in lowering molecular weight and/or inhibition of the polymerization.
Many attempts have been made to find a suitable emulsifier in place of fluorinated surfactant for such polymerizations, as disclosed in the background section of U.S. Pat. No. 6,512,063 in which sodium salt of hydrocarbon sulfonates as non-fluorinated but ionic emulsifier was employed. The ionic emulsifiers are undesired for high purity applications due to high levels extractable ions. Furthermore, alkyl sulfonates act as implicit chain transfer agent in the emulsion polymerization of fluoropolymers, as a result, it cannot be used in sufficient amount to produce small particle size latex without inhibiting such polymerizations.
Emulsifier-free aqueous emulsion polymerization process for making fluoropolymer such as TFE and/or VDF copolymers is described in WO 02/088207. In emulsifier free emulsion polymerization, first only inorganic ionic initiators such as persulfates or permangamates may work whereas organic peroxide initiators would not work. Second, the particle size of emulsifier free emulsion of fluoropolymers would be large; as a result, the shelf-life of latex would be very limited. Third, the solid content of emulsifier free latex is limited to low or moderate solids, where in fact a high solid latex is desirable in variety of commercial applications.
U.S. pat. Appl. No. 2006/0135716 describes fluoroelastomers having a Tg below −10° C. by copolymerizing perfluoropolyethers using an emulsion process. Alkyl groups on the listed surfactants exhibit high chain transfer activities in a conventional fluoropolymer emulsion when used at a sufficient amount to produce a stable latex, and as a result, the molecular weight of the resultant polymer will be considerably reduced. Thus, the properties of fluoroelastomers described in this reference are significantly different from fluoropolymers contemplated by the present invention.
The U.S. Pat. No. 6,794,550 describes a process in which fluoropolymer dispersions were synthesized in the presence of fluorinated emulsifiers. Nonionic emulsifiers were post-added to the dispersions, then a portion of fluorinated surfactant was removed by means of steam-volatilization at low pH. The disclosed process could never remove all of the fluorinated surfactant; therefore, the resultant fluoropolymer dispersion is not absolutely free of fluorinated surfactant, and a portion of the fluorinated surfactant will remain in the final dispersion. Moreover, the shelf-stability of said dispersion would be considerably reduced if not totally diminished due to heating the dispersion up to the steaming point at low pH. Further, the use of fluorosurfactants in the process, even when latter removed creates a waste stream containing fluorosurfactants and the associated environmental issues.
A method has been disclosed in WO 2005/082785 for removing fluorinated surfactant form waste water stream. The method comprises (i) adding a non-fluorinated surfactant to the waste water (ii) contacting the waste water with adsorbent particles to adsorb a portion of the fluorinated surfactant onto the particles (iii) separating the waste water and adsorbent particle. Although using fluoropolymer dispersion instead of waste water is not contemplated by inventors but can be practiced with some difficulties. The resultant fluoropolymer dispersion; however, is not absolutely free of fluorinated surfactant. A portion of fluorinated surfactant used in the first step of process will remain in the final dispersion.
“Fluorinated surfactant” and “fluoro-surfactant” as used herein means that the main surfactant chain contains fluorine atoms whereas in the present invention “non-fluorinated surfactant” means that there is no fluorine on the main chain but the terminal groups can contain fluorine atoms.
Surprisingly it has now been found that a stable fluoropolymer dispersion can be made by a process using absolutely no fluorinated surfactant and using only non-fluorinated, non-ionic emulsifiers containing segments of polyethylene glycol polypropylene glycol, and/or polytetramethylene glycol having varieties of different terminal groups and functions. The fluoropolymer dispersions produced have good latex stability and shelf-life, and are coagulum and adhesion free. These dispersions are absolutely free of fluorinated or partially fluorinated surfactant. In other words, no fluorinated surfactant is ever used to synthesize, produce, and/or post stabilize in this present invention.